1. Field of the Invention
The present invention relates to methods and apparatus for an edge film removal process, more particularly, for an edge film removal process for fabricating photovoltaic devices.
2. Description of the Background Art
Photovoltaic (PV) devices or solar cells are devices which convert sunlight into direct current (DC) electrical power. PV or solar cells typically have one or more p-i-n junctions. Each junction comprises two different regions within a semiconductor material where one side is denoted as the p-type region and the other as the n-type region. When the p-i-n junction of the PV cell is exposed to sunlight (consisting of energy from photons), the sunlight is directly converted to electricity through a PV effect. PV solar cells generate a specific amount of electric power and cells are tiled into modules sized to deliver the desired amount of system power. PV modules are created by connecting a number of PV solar cells and are then joined into panels with specific frames and connectors.
Typically, a PV solar cell includes a photoelectric conversion unit and a transparent conductive film. The transparent conductive film is disposed as a front electrode on the bottom of the PV solar cell in contact with a glass substrate and/or as a back surface electrode on the top of the PV solar cell. The photoelectric conversion unit includes a p-type silicon layer, a n-type silicon layer and an intrinsic type (i-type) silicon layer sandwiched between the p-type and n-type silicon layers. Typically, the silicon films of the photoelectric conversion unit are deposited by a plasma enhanced chemical vapor deposition (PECVD) process while the transparent conductive film may be deposited by a PECVD or a physical vapor deposition (PVD) process. As the photoelectric conversion unit and the transparent conductive film may be formed in different deposition equipment, the profile and dimension of each layer formed along the edge of the substrate may be different. For example, a PVD transparent conductive film deposition process can typically form the transparent conductive film throughout the substrate surface, and sometimes around the edge and onto a backside of the substrate. In contrast, in a CVD silicon deposition process, a shadow frame is typically utilized during deposition to cover substrate edge, thereby forming an electrical path to ground for preventing arcing and plasma leakage. However, use of the shadow frame during deposition may prevent silicon deposition along the edge of the substrate, thereby leaving edge of the substrate free from silicon films.
Accordingly, different deposition processes used for depositing the film stack on the substrate surface may often result in mismatched film profiles and film thickness at the edge of the substrate. As the film stack on the substrate periphery region are required to be removed or shaded out for the subsequent packaging or bonding process for a framing holding module, mismatched edge film profile and thickness often cause edge film removal process difficulty, thereby resulting in unwanted edge residual that may cause subsequent packaging and/or bonding process failure.
Therefore, there is a need for an improved method and apparatus for edge removal process on the substrate for fabricating photovoltaic devices.